Transcript
JULIA COOKE
Multicellular plants and animals can be complex, with many parts having different functions. An organ is a self-contained part of an organism with specific functions. And these organs usually consist of different materials or tissues. Your organs include your liver, kidneys, and skin. Leaves are an example of organs in plants. In oaks, leaves provide the energy or food for the rest of the plant. They collect light and carbon dioxide and synthesise glucose and more complex carbohydrates that can be used for growth.
In plants, leaves are often flat for maximum light interception and thin so that the light can penetrate the leaf to power the chemical reactions inside. Because leaves also lose water, some leaves growing in dry, bright conditions are small, thick, and tough to conserve water and protect against herbivores. Where water is more abundant, leaves can be thinner and softer. Across the plant kingdom, there is much variation in leaves, including in leaf size.
Amazonian waterlily leaves, from the genus Victoria, are amongst the largest simple leaves. They can have a diameter of 2.4 metres and an area of 4.5 metres squared. Some of the smallest leaves, from plants in the Wolffia genus, which are also aquatic plants, can fit through the eye of a needle. They have a length of 0.6 millimetres and an area of 0.81 millimetres squared. The leaves of an English oak are about 8 centimetres long and 15 centimetres square in area. The leaves are about 140 micrometres, or 0.14 millimetres, thick.
Organs are not static, but can change in response to environmental conditions. In oaks leaves bud, expand, photosynthesize, senesce – or die – fall, and decompose. This is a seasonal cycle within the life cycle. Each year the oak tree absorbs some nutrients from the leaves and stores them, ready to deploy these nutrients in the leaves the following year. Quercus robur loses its leaves to avoid exposing them to cold temperatures in winter when cold limits the chemical reactions and water expansion during freezing can damage cells in thin leaves. Some species make tougher leaves that can persist throughout the winter.
Every year the oak produces entirely new leaves. Over the lifetime of an oak that adds up to a lot of leaves. The number of leaves depends on the size of the tree. In this visual, we’ve given examples of how many leaves an oak produces annually at different stages of its life. It is possible to accurately determine how many leaves are on a tree by cutting it down, pulling the leaves off, and counting them. A less destructive method involves creating a 3D visualisation after many measurements with a special laser. This can also allow for a fairly accurate estimation of the number of leaves.
But regardless of life stage, every year the leaves are lost and new ones are produced in an English oak. Seasonal leaf budding and senescence in Quercus robur is triggered by changes in temperature together with light cues. The production of an entirely new canopy of leaves each year seems extraordinary, but it is these leaves that power the whole tree and provide the energy it needs for growth and reproduction.